1. Field of the Invention
The present invention relates to a printing apparatus and a data transfer method.
2. Description of the Related Art
Printing apparatuses that adopt an inkjet system for performing printing using ink are known. Such printing apparatuses are provided with a printhead and perform printing by the printhead relatively scanning over a printing medium based on raster data. In this case, the printing apparatuses can perform printing based on multiple pieces of raster data with a single scan by the printhead.
Incidentally, high quality printing modes in which high quality printing is achieved by multi-pass printing are known. In multi-pass printing, the same region of a printing medium is printed in multiple steps. In multi-pass printing, the amount of data to be printed with a single scan is limited by randomly masking the print data. For example, in the case where a single piece of print data will be completed by superimposing eight masks, raster print data is completed with eight scans. In this case, one-eighth of the print data is printed with a single scan by the printhead, and print image quality increases as the number of scans performed in multi-pass printing increases. Furthermore, with this increase in the number of scans performed, the amount of data to be printed with a single scan decreases.
In contrast to the aforementioned high quality printing modes, there are high speed printing modes in which all pieces of raster data are printed with a single scan in the case where speed is required in printing. In this case, a high amount (maximum amount) of data is printed by the printhead with a single scan.
The printhead is mounted in a casing called a carriage and performs printing by scanning over a printing medium horizontally. It is thus necessary for a controller circuit, which performs data processing, to serialize data, reduce the number of data pieces, and transfer the data through flexible wires to the printhead.
Here, technology for transferring data to the printhead at high speed without increasing the clock speed of serial data and without increasing the number of signal lines is known. With this technology, signal lines used to access a non-volatile memory in the printhead are exclusively used also for printing operations, and therefore the signal lines for access to the non-volatile memory are switched to and used as signal lines for data transfer (see Japanese Patent Laid-Open No. 2004-090262).
Technology is also known in which in the case where a printhead having different printhead lengths for each color has a circuit configuration formed in accordance with the amount of data transferred for the longest printhead length, images of multiple colors having shorter printhead lengths are superimposed and transferred at a single time (see Japanese Patent Laid-Open No. 07-089137). In this case, the number of signal lines can be reduced.
A method for serializing data before transfer is generally used in order to reduce the number of data pieces, but if the data amount per column increases, it is necessary either to increase the transfer clock speed or to reduce the scanning speed of the carriage to such an extent that data transfer ends on time.
At this time, in the case of a high speed printing mode in which the number of scans performed is small, a single piece of serial data contains a large number of bits at which ink is discharged (a large number of dots printed), whereas in the case of multi-pass printing mode in which multiple scans are performed, the amount of data in a single piece of serial data is reduced and becomes almost “zero” (that is, there is no data). For example, in the case of using a printhead that divides a single column of data into multiple blocks and discharges ink, if 16 bits (print dots) are discharged simultaneously for each block, one-eighth thereof is the amount of data to be printed for each pass in an 8-pass printing mode. That is, data is contained at most in two dots out of 16 dots.
Furthermore, in the case of high quality printing modes in which multiple scans are performed, it is necessary to shorten discharge intervals in the scanning direction (column direction) (that is, to increase the discharge frequency) and to perform printing at higher resolution. Therefore, if the data transfer will not end on time, the scanning speed of the carriage needs to be reduced. This produces conflicts between the need to shorten data transfer intervals and the fact that data transferred itself contains almost no data.